A wheel bearing apparatus is generally known that can support a wheel of a vehicle with respect to a suspension apparatus and incorporates a rotational speed detecting apparatus to detect rotation speed of a wheel of the vehicle to control the anti-lock braking system (ABS). Such a bearing apparatus generally includes a rotational speed detecting apparatus with a magnetic encoder having magnetic poles alternately arranged along its circumference. A sealing apparatus is integrated and arranged between the inner and outer members that rotate relative to each other via rolling elements (balls). A rotational speed detecting sensor is present to detect the variation in the magnetic poles of the magnetic encoder according to the rotation of the wheel.
The rotational speed sensor is usually mounted on a knuckle, forming part of the suspension apparatus, after the wheel bearing apparatus is mounted on the knuckle. Recently proposed is a wheel bearing apparatus incorporating a rotational speed detecting apparatus with a rotational speed detecting sensor that is incorporated into the wheel bearing. This apparatus is to reduce the size of the wheel bearing apparatus as well as to eliminate troublesome in air gap adjustment between the rotational speed sensor and the magnetic encoder.
An example of a known wheel bearing apparatus incorporating a rotational speed detecting apparatus is shown in FIG. 8. The wheel bearing apparatus incorporating a rotational speed detecting apparatus comprises an outer member 51 and an inner member 52. The outer member 51 forms a stationary member adapted to be secured to a knuckle (not shown). The inner member 52 is inserted into the outer member 51 via double row balls 53, 53. The inner member 52 includes a wheel hub 55 and an inner ring 56 fit onto the wheel hub 55.
The outer member 51 outer circumference has an integrally formed body mounting flange 51b. The outer member 51 inner circumference has double row inner raceway surfaces 51a, 51a. On the other hand, the inner member 52, i.e. the wheel hub 55 and the inner ring 56, have, on their outer circumferences, double row inner raceway surfaces 55a, 56a, respectively, opposing the outer raceway surfaces 51a, 51a of the outer member 51. The inner ring 56 is press-fit onto a cylindrical portion 55b that axially extends from the inner raceway surface 55a of the wheel hub 55. Double row balls 53, 53 are contained between the outer and inner raceway surfaces and are held by cages 57, 57.
The wheel hub 55 is integrally formed with a wheel mount flange 54 for mounting a wheel (not shown) on its outer circumference. The end of the cylindrical portion 55b is plastically deformed radially outward to form a caulked portion 58 to axially secure the inner ring 56. A seal 59 and a sensor cap 63 are mounted on the ends of the outer member 51 to prevent leakage of lubricating grease sealed within the bearing and to prevent entry of rain water or dust from the outside into the bearing.
A magnetic encoder 60 is press-fit onto the outer circumference of the inner ring 56. The magnetic encoder includes an annular supporting member 62 formed by a magnetic metal sheet. The member 62 has a substantially L-shaped cross-section. An encoder body 61 is adhered to one side of the annular supporting member 62. The encoder body 61 is formed of a rubber permanent magnet mingled with ferritic powder and has N and S poles alternately arranged along its circumference.
The sensor cap 63 is fit into the inner circumference of the inner-side end of the outer member 51 to close the opened portion of the outer member 51. The sensor cap 63 includes a cylindrical cap body 64, with a bottom formed by plastic injection molding, and an annular metallic member 65 joined to the cap body 64. The annular metallic member 65 is press-formed of a steel sheet to have a substantially L-shaped cross-section. The annular metallic member 65 is integrated with the cap body 64 during injection molding of the cap body 64.
An axially projecting portion 66 is formed on the radially outer portion of the cap body 64. The portion 66 includes an inserting bore 67 at a position corresponding to the magnetic encoder 60. A sleeve 68 is fit into the inserting bore 67. A sensor 69 is inserted into the sleeve 68 via an O ring 70 as shown in FIG. 9. The sensor 69 includes a magnetic detecting element 71, such as a Hall element, a magnetic resistance element (MR element) etc. changing characteristics (magnetic polarities) in accordance with the flowing direction of magnetic flux, and an IC etc., incorporating with a waveform shaping circuit for shaping the output waveform of the magnetic detecting element 71. The sensor 69 forms part of the ABS of an automobile for controlling the rotational speed of wheels while detecting it.
A mounting piece 72 is mounted on the sensor 69 and projects from it. The mounting piece 72 has a bolt inserting aperture 73 that receives a sleeve 74. Thus, the sensor 69 can be mounted on cap body 64 by screwing a sensor securing bolt 75 inserted into the sleeve 74 into an insert nut 76.
The cap body 64 and the sensor 69 are formed of synthetic resin material (plastic) of low water absorption of the groups, such as PA (polyamide) 612, PPS (polyphenylene sulfide) etc., in order to keep the sealability while preventing the dimensional change and the generation of cracks due to water absorption of the sensor mounting members (see Patent Document-Japanese Laid-open Patent Publication No. 2007-120560).
In such a wheel bearing apparatus incorporating a rotational speed detecting apparatus of the prior art, the cap body 64 is formed by injection molding of synthetic resin fed into a mold through a gate 77 as shown in FIG. 10. An injecting port to the gate 77 has a conical cross-section as shown by a two dotted line in FIG. 10(a) and is formed integrally with a mold body. Accordingly, the gate 77 should be arranged at a position apart from the projected portion 66 in order to prevent the injecting port from being interfered with by the projected portion 66 of the cap body 64.
However, the synthetic resin tends to circumferentially flow along the fitting portion, as shown by dotted line arrows in FIG. 10(b), if the gate 77 is positioned away from the center of the cap body 64. Thus, although the synthetic resin smoothly flows circumferentially in regions A, it would flow slowly and stay in a region B. Such a difference in flowing speeds of synthetic resin causes differences in time duration of solidification of the synthetic resin.
In other words, the synthetic resin tends not to be easily shrunk in the regions A where the synthetic resin can flow smoothly. On the contrary, the synthetic resin tends to be easily shrunk in the region B where the synthetic resin cannot flow smoothly. In such a case, shrink marks etc. would be caused in the region B. Thus, the cap body 64 would be deformed to an oval-like configuration of less roundness. Accordingly, it is believed that the sealability of the fitting portion of the cap body 64 would be detracted. Thus, foreign matter, such as rain water or dust, would enter into the bearing. The sensor cap 63 would move or slip relative to the outer member 51 by shocks or vibrations caused during running of the vehicle. Thus, the initially set air gap between the magnetic encoder 60 and the magnetic detecting element 71 would be detracted.